ORIGIN OF LYMPH. 303 



Lymph coagulates more slowly than the blood, and the clot is less 

 firm. Urea is present to a greater amount than in plasma. 



Histologic Composition of I/ymph. Examined under 

 the microscope lymph is found to contain colorless cells, lympho- 

 cytes, which pass into the blood with the lymph and there become 

 leukocytes (p. 291). Fat-globules are also found, especially after 

 a meal, and in the lymph from the thoracic duct. 



Origin of I/ymph. While there is no doubt that the source 

 of the lymph is the blood, still there is a difference of opinion as 

 to the manner in which it escapes from the blood-vessels. 



Theory of Ludwig. Ludwig believed that the pressure of the 

 blood in the capillaries was sufficient to cause the plasma to filter 

 through their walls, thus forming the lymph. He also believed 

 that diffusion played a part in lymph-formation. He expressed 

 his views as follows : " The blood which is contained in the vessels 

 must always tend to equalize its pressure and its chemical con- 

 stitution with those of the extravascular fluids, which are only 

 separated from it by the porous blood-vessel walls. If, for ex- 

 ample, the quantity of blood in the vessels has increased, the mean 

 blood-press.ure is also increased, and at once a portion of the blood 

 is driven out into the tissues by a mere process of filtration. The 

 same result is brought about when the constitution of the blood is 

 altered by the absorption of food or by increased excretion by the 

 kidneys, blood, or skin, or when the composition of the tissue- 

 fluids is altered in consequence of increased metabolic changes 

 taking place in the tissues. In the latter case the changes 

 brought about in the lymph are effected by processes of diffusion." 

 This theory of Ludwig may, therefore, be termed that of filtration 

 and diffusion. 



Theory of Heidenhain. This experimenter studied the subject 

 of lymph-formation by examining the flow from the thoracic 

 duct. He found that if the thoracic aorta was obstructed, there 

 would follow a fall of arterial blood-pressure below the obstruc- 

 tion, and yet there was no diminution in the flow of lymph in the 

 thoracic duct, and in some cases it was increased. If lymph was 

 formed by filtration from the blood, a diminution of blood-pressure 

 should have been followed by a corresponding lessening of lymph- 

 production. Other experiments demonstrated that the flow of 

 lymph might be increased without correspondingly increasing the 

 blood-pressure. He also found that if commercial peptone and 

 some other substances were injected into the blood-circulation, the 

 amount and concentration of the lymph would be increased, al- 

 though blood -pressure might be reduced ; also that if concentrated 

 solutions of sodium chlorid or sugar were injected, the flow of 

 lymph would be increased and its concentration diminished. If 

 blood-pressure was increased, it was so but slightly. Heidenhain 

 terms substances having the power of increasing the flow of lymph 

 lymphagogues. These experiments demonstrated that the lymph 



